Cathode-coupled wide-band amplifier



Patented Feb. 8, 1949 UNITED 9 9 7 a CAT'H0DEG0UPLEDWIDE-BAND AMPLIFIERi Alfred C- qhmefltt .E rvi l a-i e iemr is fiadiflfiqrljgration ofAmerican corporation of Dela a e Application December 28, 19.44,SerialNo, 579,176 6 iii-1.7

and low impedance circuits. However, in order to obtain the requiredgain with low impedance circuits, it is necessary'that high Gm(transconductance) tubes be used. For certain applications, such as theinput stages of wide-band, high-gain receivers, triodes are moredesirable 1 amslificeti O c were; whereas? of input signal voltages. Phstd-f a ures ch a eris i 0? y invar s t or h h art y 1.1 t e a s da caim 1 i i t ti li wt er th t i er mze a an mq l i 2 ticn together withfurther objects and a,

. .a 's W111 b h Re eemed hr r b 1 irate y rio us modifications oi e} ieeeie li li t ar than hentodes because of their inherently lower I noisecharacteristics. A low impedancecircuit frequently used is the cathodeoutput or cathode follower circuit. It has been proposed to use acircuit of this type in conjunction with a con vent al or u de h d meifi r bu such a circuit, besides requiring the same number of circuitelements as two amplifier stages of cone t nal e ha h ret en us d ipentode tube as the grounded-cathode arnplifier.

In some cases this dual stage does not provide adequate stability, andalso does not provide better noise characteristics than conventionalcircuits employing pentode tubes.

.;. have c r d hat by c m nin a athod out u a d ca hod p age us n a Sintwin-triode tube, a high-gain, wide-band'arne pli er Stage is obta edhiQ mpares my??? a y Wi h a ipentode sta w reaped t in stability andeconomy, while it has far s noise characteristics. The new oincui havecalled the cathode-coupled i' WlHt-iihlijfld amplifier, provides greaterflexibility than conventional amplifier circuits, and can be usedfor IF,yideo, comer-tenor detector s erv1 s. It is accordingly an object of thepresent invention to provide a wide band, stable amplifier having bettersignal-to-noise ratio inulti: grid tube amplifiers.

A u e Me o th inv nti i to pmr e an amplifier that has output to inputshielding comparable to that of a screen grid or pentode tube but withrelatively low-er input capacity:

A more specific object of the invention is to amide, air of aihoourldtriosieomnlh A n T i wrmeeis an ai inn I1 1 1* 8 5 hi h s. d era iQ li th its. -1 arid lea e sonn t-r I}??? or IF tier minals t.cathode-coupled twin-. a I her n ee e-ha it 5. d t i f e'l m- Pl fi 9%?a i liitll stew ii .rer 11 which case the input terminalsl be connectedto the output oi the converter stage.

The cathode 5 of section "I -1 is connected to input terminal 4 orground through a radio frequency (RF) choke .1 n 8 9 reeiieeh connectedfor high irequencies to ground by way of condenser 9, the aglode b eingenergized from a source of direct current represented by B+.

grid amplifier. tion, like the cathode of (the first section, isconnected to the high potential endof coil 1 so that 3? .991} is th oiitput load impedance in e. or 21" the first l i i the dance'of thecathode circuit'of the form t v 0 sections.

has connected to it a load impedance, shown by way of example as aprimary 13 of an output transformer T0, the secondary M of which feedsan RF (or IF) stage l which may be similar to the stage just describedemploying tube l. The condenser !6, shown dotted, since it may representonly the distributed capacity of winding l3, strays and tube capacities,serves to tune the output circuit of the stage to the center frequencyof the band of frequencies to be amplified, which may, for example, be 4to 6- maimegacycles) wide. Resistor l7 shunted across the transformersecondary It plus the plate resistand by-pass capacitor customarilyrequired in a screen supply are eliminated. Since the plate currentsin'the two triode-units swing in opposite ance of T2 serves to load thetransformer to give the proper band width.

The cathode coupling coil 7 should have an impedance that is highcompared tothe internal 1 input impedance of the grounded-grid stage '1:

over the frequency band to be amplified and serves to eliminate theapparent reactive (capacitive) component in the cathode circuit byparallel resonance with the distributed cathode to ground capacity l8,thereby improving the signal-to-noise ratio and .the gain, Because ofthe low cathode input impedance of T1 and T2 this resonant circuit has avery low Q and hence a very wide band. This makes the value of the chokeuncritical. I have used one of approximately microhenries with verysatisfactory results.

It is important for the proper operation of the circuits describedherein that the grid bias and plate voltage be selected of such valuesthat no limiting or clipping takes place. Although in Fig, 1, the gridsof both sections operate at zero bias, the plate voltage is chosen ofsuitable value to avoid the limiting action mentioned.

The circuits of Figs. 2 and 3 are fundamentally similar to the circuitof Fig. 1 except that in Fig. 2 there is included in the cathodecircuit, in series with the high frequency choke, the usual se1fbiasingresistance and shunt condenser network 19 for varying the gain of theamplifier, and in Fig. 3 the gain may be varied by changing the D. C.voltage on either, or both, of the grids by means of a voltage derivedfor example from an A. V. C. system, as is well known in the art.

With a type 6J6 tube I have used 70 volts on the plate with the grids at0 and no cathode bias (Fig. 1), and about 125-150 volts on the platewith the grids at O, and about a 50-ohm cathode resistor unbypassed(Fig. 2).

The gain of the wide-band amplifier of the cathode-coupled twin-triodetype herein described is equal to by reason of degeneration of the gridto cathode capacity, Gm being the transconductance of each tube section,C1 and Co the input and output capacities, respectively Tp the plateresistance, and Aw being equal to 21A The advantage of a lower capacityis that since the total capacity across the circuit is what limits thegain times directions, the total plate load tends to remain uniform, andsubsequent similar stages have little influence on each other such asmight result from varying'load on the plate supply.

While I have shown and described a preferred embodiment of my invention,it will be understood that various modifications and changes will occurto those skilled in the art without departing from the spirit and scopeof this invention. I therefore contemplate by the appended claims tocoverany such modifications as fall Within the true spirit and scope ofmy invention.

What I claim is:

1. Amide-band high-frequency amplifier comprisinga cathode-followerstage coupled to a cathode-input, grounded-grid stage, the couplingmeans between said stages being constituted by an inductance having animpedance at the frequencies to be amplified which is high compared tothe internal input impedance of the groundedgrid stage.

2. A wide-band high-frequency amplifier comprising a cathode-followerstage coupled to a cathode-input, grounded-grid stage, the couplingmeans between said stages being constituted by a high frequency chokeconnected between the cathodes of both stages and ground, said chokeresonating with the capacitance between the cathodes and ground over awide range of frequencies.

3. A wide-band high-frequency amplifier comprising a cathode-followerstage coupled to a cathode-input, grounded-grid stage, the couplingmeans between said stages being constituted by an inductance which iscommon to the cathode circuits of both' stages, the value of saidinductance being high compared to the input impedance of thegrounded-grid stage at the frequencies to be amplified and being of theorder of 20 microhenries.

4. A circuit for the amplification of a wide band of high frequenciescomprising first and second electron discharge paths each constituted bya cathode, a grid and an anode, a source of high frequencies connectedbetween the grid of the first discharge path and ground, a commonreactive impedance connected between said cathodes and ground, a groundconnection to the grid of the second discharge path, an output impedanceconnected between the anode of the second discharge path and a source ofanode potential, and a high frequency ground connection to the anode ofthe first discharge path, the biases on the grids and the potentials onthe anodes being chosenof such values that the circuit functions as'alinear amplifier over a wide range of input signal voltages;

5, Acircuit for the amplification of a wide band of high frequenciescomprising first and second electron discharge paths each constituted bya cathode, a grid and an anode, a source of high frequencies connectedbetween the grid of the first discharge path and ground, a commonreactive"impedance connected between said oathodes and ground, a groundconnection to the grid of the second discharge path, an output impedanceconnected between the anode of the second discharge path and a source ofanode potential, V

and a high frequency ground connection to the anode of the firstdischarge path, said common cathode impedance having a value that ishigh compared to the input constituted by the grounded grid and itscathode, over the band of frequencies to be amplified.

6. A circuit for the amplification of a wide band of high frequenciescomprising first and second electron discharge paths each constituted bya cathode, a grid and an anode, a source of high cathode impedancehaving a value that is high compared to the input constituted by thegrounded grid and its cathode, over the band of frequencies to beamplified, and the biases on the grids and the potentials on the anodesbeing chosen of such values that the circuit functions as a linearamplifier over a wide range of input signal Vo1t-. ages.

ALFRED C. SCI-IROEDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,068,112 Rust Jan. 19, 19372,162,878 Brailsford June 20, 1939 2,215,796 Rust et a1 Sept. 24, 19402,270,012 Shepard Jan. 13, 1942 2,276,565 Crosby Mar. 17, 1942 2,315,040Bode Mar. 30, 1943 2,330,109

Brown Sept. 21, 1943

